1. Field of the Invention
The present invention relates to an electron beam image drawing device for forming a fine pattern on a semiconductor substrate by means of an electron beam; in particular, it relates to a method of manufacturing an image drawing mask employed in an electron beam image drawing device of the integrated-pattern (cell projection) type whereby a fine pattern can be formed at higher speed.
2. Description of the Related Art
With progress in LSI techniques, there have been rapid advances in increasing the fineness of circuit patterns formed on semiconductor devices. One method of forming fine patterns is image drawing using an electron beam. Such an electron beam image drawing system is a system that is effective for forming fine patterns of width less than 0.25 .mu.m which are necessary in order to achieve even higher densities of integration of semiconductor circuits.
As an electron beam image drawing device, a device as shown in FIG. 10 may be employed. The electron beam image drawing device shown in FIG. 10 comprises a first aperture 103 formed with a rectangular aperture and an EB mask 106 formed with a plurality of basic aperture patterns 106A. This first aperture 103 and EB mask 106 have the function of forming an electron beam 150A into an electron beam 150B of a prescribed pattern. In particular, complex patterns can be drawn by a single exposure (integrated-pattern image drawing) by means of a basic aperture pattern 106A of EB mask 106. Electron beam 150B is directed on to a semiconductor substrate 111 covered with a resist (not shown), so that a fine pattern is formed in this resist.
As described above, in an integrated-pattern type electron beam image drawing device, an EB mask 106 is employed that is provided beforehand with a plurality of basic aperture patterns 106A. The basic aperture pattern 106A formed on this EB mask 106 was constituted by selecting a single cell portion of an identical pattern from among the fine patterns to be formed on semiconductor substrate 111. Conventionally, basic patterns were simply selected in the order of greatest repetition number from the fine patterns to be formed on semiconductor substrate 111. Several basic patterns were then determined, from the plurality of selected basic patterns, such that the number of shots needed to draw the entire semiconductor chip should be as few as possible. A basic aperture pattern 106A corresponding to the basic patterns that were thus determined was then formed on EB mask 106.
Also, it was necessary to select basic patterns such that basic aperture pattern 106A should contain as many fine patterns as possible, in order to reduce the number of shots when image drawing the entire semiconductor substrate 111. A plurality of types of basic aperture pattern 106A were therefore formed in the size allowed in EB mask 106. However, the above prior art example was subject to the following inconveniences. Specifically, when the aperture area of the basic aperture pattern 106A formed on EB mask 106 gets large, beam blur is produced, which lowers the image drawing quality (resolution) of the fine pattern. The reason for this is that the total beam current of electron beam 150B passing through EB mask 106 becomes large, resulting in a Coulomb effect being generated due to mutual repulsive force of the electrons contained in the electron beam.
FIG. 11 shows the relationship between beam current of the electron beam and beam blur. As can be seen from this Figure, when the beam current gets large, beam blur of electron beam 150B becomes large, so formation of a fine pattern on semiconductor substrate 111 becomes difficult. With the conventional method of selecting a basic aperture pattern, no consideration at all was given to the aperture area of basic aperture pattern 106A formed on EB mask 106. It was therefore not possible to suppress the deterioration of image-drawing quality caused by the Coulomb effect.